The present invention relates to a method and system of generating an eccentricity compensating signal of the type used in either a gauge meter or a position control scheme for a rolling mill installation and it will be described with particular reference thereto; however, it has much broader applications and may be used in other types of rotary equipment and in various other systems for eccentricity compensation in a rolling mill. Indeed, the invention may be employed in other manufacturing processes wherein there is to be compensation for a periodic force fluctuation correlated to or created by a rotary element.
In hot and cold rolling mills the eccentricity of the backup roll or rolls causes substantial difficulties, one of which is variation in the gauge of the strip being rolled. This is caused by a change in the opening between the working rolls during the processing of the workpiece, work or strip. This problem is becoming more pronounced as the specification for strip thickness from a rolling mill becomes more stringent. Indeed, competition in such industries as the steel industry has been devastating and mills seek orders on the basis of price and dimensional stability of metal strip. This accentuates the need for precise control which is difficult to obtain with massive, somewhat imprecise machines such as rolling mills. Also, some tolerance specifications have a tendency to preclude existing mills from consideration because of the inability to deal with roll eccentricity. There is a tremendous demand for a system to allow existing mills (purchased when speed was the basic requirement) to be used in the present market where speed must be accompanied by extreme uniformity. The many proposed eccentricity control systems have not met the need. Indeed, they generally anticipate a new mill with little eccentricity problems.
When the backup rolls are several feet in diameter and must be periodically reconditioned by a grinding process, surface undulations and/or eccentricities are unavoidable. Since most mills include two backup rolls engaging the outer surfaces of the work rolls, the eccentricity of both backup rolls causes variations in the strip gauge thickness, which variation may be in phase or out of phase. Indeed, even if in phase, slippage or other variations can cause the strip rolling variables caused by the surface of the backup rolls to become angularly displaced.
Because of the variables caused by eccentricity and other surface variations of the backup rolls, rolling mills often employ some type of position control or automatic gauge control added to the normal system for controlling the rolling mill. These systems attempt to compensate for fluctuations in the delivered gauge caused by rotational variations in the backup rolls. In many of these systems, the mill is adjusted for a normal run and the position control, gauge control or gauge meter system monitors and corrects for gauge errors or force variations during the actual rolling operation. These control systems generally employ some type of feedback loop to sense variations in some parameter and to take corrective actions. When using a gauge meter, the force signal from a load cell is monitored as an indication of gauge variation. As the gauge increases, or a harder surface is presented at the roll opening, there is an increase in the force exerted by the backup roll against the work roll. This increased force is sensed by the gauge meter and signals for a change in the displacement of the rolls in a direction to increase the roll force further to establish the proper gauge. The reverse of this occurs if the gauge or thickness increases or softer material is presented to the roll. The same general arrangement is employed for position control; however, it does not generally require consideration of the modulus of the material which is indicative of harder or softer material being processed through the rolling mill. In either system, eccentricity of the backup roll produces a periodic increase and decrease of the roll force as the rolls rotate. When the eccentricity causes an increase in the roll force, without any compensation, the automatic gauge control interprets this condition as an increase in the gauge or material hardners. This is not true. Consequently, a signal to increase the applied force is created. This signal compounds the errors in delivery gauge caused by roll eccentricity. The reverse occurs when eccentricity causes a decrease in roll force being measured by a load cell. These shortcomings are well known in the art of operating rolling mills. A substantial number of techniques has been employed to overcome the persistent problems created by backup roll eccentricity and the demand of the industry for tighter tolerances of the delivered strip. Systems which could theoretically operate in accordance with prior product tolerances are now not considered as viable systems to obtain the required tolerance control on a massive rolling mill.
The patents incorporated by reference into this specification illustrate the general type of systems employed for compensation of the force variations caused by backup roll eccentricity. Some of these systems are predictive in nature. In that situation, the work rolls are forced together and a force reading for one or more revolutions of the backup rolls is recorded. This is considered background data for eccentricity compensation. These systems are not successful. For instance, the backup rolls can be shifted with respect to each other due to differences in outer diameters, slippage or other variables between two backup rolls. This existing condition ultimately destroys the background force pattern of predictive systems. Another manner of attacking the complex problem of eccentricity in rolling mills is the use of a system which periodically stores a bulk of information and processes it in a Fourier processor. This processor produces a spectrum which is employed for eccentricity compensation. As can be seen, these continuously operating systems require an accumulation of data before any action can be taken on eccentricity; therefore, there is a substantial time lag between variations and actual correction. This type system continuously processes eccentricity by memorizing the variations and updating a control system. The predictive and memorized data concepts, although they can theoretically be of assistance in the problem of gauge control to eliminate eccentricity variations, have not been successful and are not now employed successfully in the rolling mill art. This fact can be explained by the massive equipment and gauge control demands for current product. Consequently, there is still a tremendous demand for a system which will compensate rapidly for in-process variations caused by eccentricity of the backup rolls during the actual processing of the strip, irrespective of changes in the strip modulus, input gauge and other factors employed in both position control, automatic gain control and gauge meter systems. In view of the deficiencies and costs of prior compensating systems, whether analog or digital, rolling mills still generally employ only gauge meters and position controls without effective eccentricity compensation and with a product that is often out of specification.
Mechanical devices have been attempted as low cost arrangements for backup roll eccentricity. Another suggestion, which has been made for solving the problem of roll eccentricity, is the provision of a filter for passing a signal including both the general steady state force and eccentricity force components. By adjusting the filter with a pass band generally centered around the roll frequency and providing a high Q factor, the output of these filters can be an approximation of the eccentricity force component. These filters, both analog and digital, are not accurate enough. The frequency can vary so that the Q must be enlarged to allow normal operation. When this occurs, there is no precise signal passing the filter. To allow a more accurate filtering process, it is suggested that the force measured by the load cell, which includes both the steady state force component and eccentricity force component, can be multiplied by either a sine or cosine of the backup roll rotation. By then centering the pass band with respect to the frequency of the sine wave caused by rotation of the backup roll, a more precise separation can occur between steady state component and the eccentricity component of the force being measured or monitored from the load cell. These forward pass band filters, digital or otherwise, are generally shown in Ichiryu U.S. Pat. No. 4,036,041. This patent also describes the difficulty with pass band filtering concepts. The pass band and the center of the band are controlled only from history and there is no feedback through the filtering loop itself. This type of system is generally employed with the standard BISRA-AGC gauge meter formula which was developed to exclude from gauge control the constantly variable, generally impercise material modulus. Thus, the systems are generally not applicable to the position control wherein material modulus is a factor. Consequently, these gauge meter systems must be manually adjusted for each material and for its prior processing.
As a summary, many patents have been obtained and many more systems have been suggested for removing eccentricity in the proper algebraic relationship from a rolling mill for the purpose of precise gauge control. Generally, the tolerances have decreased more rapidly than obtainable precision has increased in these systems. Consequently, at this time there is still a demand for an accurate, continuous, low cost and durable system for removing eccentricity variations from the control of the thickness of metal being processed by a rolling mill. In addition, the system must be applicable to control systems other than the standard gauge meter which has less application to the rolling mill art. The system must be fast operating and responsive in a small rotational angle of the backup roll.